<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0,user-scalable=no">
    <title>引入不同css</title>
    <!-- <link rel="stylesheet" media="媒体类型 and|not|only (媒体特性)" href="css位置"> -->
    <!-- /* 屏幕小于640 一行显示一个div */ -->
    <link rel="stylesheet" media="screen and (max-width:639px)" href="不同css文件/smaller640px.css">


    <!-- /* 屏幕大于等于640 一行显示2个div */ -->
    <link rel="stylesheet" media="screen and (min-width:640px)" href="不同css文件/bigger640px.css">
</head>

<body>
    <div>1</div>
    <div>2</div>
</body>

</html>